Handle CPACK_MONOLITHIC_INSTALL in some rare use cases.
For example, when CPACK_<GEN>_COMPONENT_INSTALL and CPACK_MONOLITHIC_INSTALL are both set. Previously, this combination of variable settings produced an error without any explanation. Now, in this case CPACK_MONOLITHIC_INSTALL wins without trouble. This is useful for when e.g. CPACK_ARCHIVE_COMPONENT_INSTALL is globally on and one wants MONOLITHIC install for STGZ (but not other generators). The same behavior may be obtained by re-setting CPACK_ARCHIVE_COMPONENT_INSTALL to 0 but in any case the 'both set' case should have been handled without error.